Process for the production of surfactant mixtures based on ether sulfonates and their use

ABSTRACT

A process for the production of a surfactant mixture for use in enhanced oil recovery wherein the surfactant mixture comprises an alkyl ether sulfonic acid or salt thereof as a principal constituent and at most a substantially equal quantity of alkoxylated alcohol. The surfactant mixture is prepared by reacting an alkyl ether sulfate with a stoichiometric excess of an alkali metal sulfite solution at about 160° C. to 220° C. under mildly alkaline pH conditions, and then extracting sulfate salt therefrom using a substantially water-insoluble alcohol at a lowered temperature.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a new process for the production ofsurfactants and surfactant mixtures based on fatty alcohol ethersulfonates which enables this industrially interesting class ofsurfactants to be economically obtained.

2. Discussion of Related Art

Enhanced oil recovery, in which the quantities of residual oil retainedin the reservoir by viscosity and capillary effects in the pore spaceare made at least partly recoverable, is acquiring increasing interest.Many types of process have been proposed for enhanced oil recovery,including for example polymer flooding, alkali flooding, thermalprocesses or solution flooding.

The present invention is concerned with the process known as surfactantflooding. Whereas, in the fresh oil reservoir, the oil is present ascontinuous phase in the rock pore space, the oil phase disintegrateswith increasing primary and secondary recovery into individual discretedroplets which are retained in narrow pores under the effect of the highinterfacial tension. Overcoming the capillary forces either requiresextremely high pressures or a considerable reduction in the interfacialtension between water and oil using suitable surfactants. In eachindividual case, this reduction depends to a very large extent on thereservoir temperature, the salinity of the reservoir water and thecomposition of the oil. The desired so-called middle phasemicro-emulsion--a central third phase between the heavier salt waterphase and the lighter oil phase--is only formed through adaptation ofthe particular surfactant mixture and under the conditions of theextreme reduction in interfacial tension. The formation of the middlephase micro-emulsion is crucially important to the recovery of residualoil.

Suitable surfactants have to satisfy various requirements over and abovetheir ability to reduce interfacial tension to a considerable extent.For example, they must not form any deposits in the flood water orformation water because otherwise there would be a danger ofirreversible blockage in the reservoir. They should be adsorbed to therock to only a minimal extent, if at all. They should be stable underreservoir conditions, remaining stable for periods of 1 to 3 years fordistances between the injection probe and the recovery probe of 50 to300 m and flooding rates of approx. 0.3 m/d.

Ether sulfates, ether carboxylates, ether sulfonates and etherphosphates in particular have been proposed as surfactants for use inhigh-salinity reservoir waters. Ether sulfates and ether phosphates arereadily obtainable on an industrial scale, but lack stability tohydrolysis. Ether sulfonates and, more particularly, alkyl ethersulfonates have shown particularly interesting properties in screeningtests. They combine high electrolyte compatibility with high stabilityto hydrolysis at high temperatures. In particular, however, they alsoshow the appearance of the desired three-phase states in theoil/water/surfactant systems with a broad middle phase micro-emulsionrange and a pronounced reduction in interfacial tension. Mixtures ofthese surfactants with other surfactants and/or so-called co-solventsopen up technically interesting possibilities in the field of enhancedoil recovery.

Numerous syntheses have been proposed for the preparation of ethersulfonates, cf. for example the Article entitled"Fettalkoholethersulfonate fur die tertiare Erdolforderung (FattyAlcohol Ether Sulfonates for Enhanced Oil Recovery)" inFette-Seifen-Anstrichmittel, 1985, 382-385 and the literature citedtherein. The problem addressed by the present invention was to provide atechnologically simple process which would enable this interesting classof surfactants to be economically produced and which, in particular,would provide as a direct reaction product a surfactant-containingmulticomponent mixture which would have valuable properties as such forthe stated application. This primary surfactant mixture would consist ofthe above-mentioned alkyl ether sulfonates as the desired main componentand, in addition, would contain nonionic fatty alcohol ethers formingthe basis of the ether sulfonates and, optionally, so-called co-solventsbased on synthetic and/or natural alcohols.

DESCRIPTION OF THE INVENTION

Other than in the operating examples, or where otherwise indicated, allnumbers expressing quantities of ingredients or reaction conditions usedherein are to be understood as modified in all instances by the term"about".

In a first embodiment, therefore, the present invention relates to aprocess for the production of surfactant mixtures based on alkyl ethersulfonic acids or salts thereof (ether sulfonates) as principalconstituent which may be used in particular in enhanced oil recovery andare obtained by reaction of alkyl ether sulfates with an aqueous alkalimetal sulfite solution at temperatures of 160° to 220° C., this processbeing characterized in that, to prepare mixtures--at least substantiallyfree from sulfate salt--of the ether sulfonates based on alkoxylatedalcohols of natural and/or synthetic origin with at most substantiallyequal quantities of the alkoxylated alcohols, the reaction is carriedout with a stoichiometric excess of alkali metal sulfite in aqueousmedium at a mildly alkaline pH value, the reaction mixture is optionallyleft to continue reacting under the described reaction conditions andthe surfactant mixture formed is extracted with at least substantiallywater-insoluble alcohols at reduced, but still elevated temperatures.

In another embodiment, the invention relates to the surfactant mixturesproduced by this process, more particularly in admixture with residualalcohol from the extraction stage for tertiary oil recovery.

It is known that the alkoxylation of alcohols of natural and/orsynthetic origin, particularly fatty alcohols, with ethylene oxideand/or propylene oxide gives correspondingly alkoxylated fatty alcoholcompounds resembling nonionic surfactants in character. Their terminalesterification with sulfuric acid to form the corresponding sulfatesalts gives the fatty alcohol sulfates.

In the type of process with which the present invention is concerned,the alkyl ether sulfates are reacted in a nucleophilic substitution withsulfite to form the alkyl ether sulfonate which is referred to moresimply hereinafter as "ether sulfonate". The associated reactionequation of the desired main reaction is as follows:

    R'--O--SO.sub.3 Na+Na.sub.2 SO.sub.3 →R'--SO.sub.3 Na+Na.sub.2 SO.sub.4                                                  (I)

In this equation, R' corresponds to the alkoxylated alcohol radical.

The nucleophilic substitution in question requires comparatively hightemperatures and correspondingly high pressures, but is nevertheless acomparatively slow reaction. This reaction, which is carried out inaqueous medium, results in the formation of considerable quantities ofalkoxylated alcohol as saponification product, normally resembling anonionic surfactant in character. At the same time, however, thefollowing additional consideration is crucial to the application of thesurfactants formed in accordance with the invention:

The sodium sulfate accumulating in aqueous phase as a secondary reactionproduct must have been almost completely removed from the surfactantmixture to be ultimately separated off. If, in practical application,substantial quantities of the sulfate anion were to be introduced intothe oil reservoir through the surfactant components thus formed, thecapillary system would become rapidly blocked by the in situ formationof alkaline earth metal sulfates. Accordingly, the type of reaction inquestion for the formation of the ether sulfonates involves a very muchmore complex problem for the practical and economic production ofsuitable surfactants or surfactant mixtures than originally appeared tobe the case. The situation is additionally complicated by the fact thatthe reaction of the ether sulfates with sodium sulfite in aqueoussolution is not without problems. The reaction mixtures tend to gel sothat they are difficult to handle and, at the high reaction temperaturesrequired, quickly form crust-like deposits on the inner walls of thereactor so that the reaction has to be interrupted to clean the interiorof the reactor.

The teaching according to the invention as described in the followingencompasses two problem areas which are solved by the measures taken inaccordance with the invention in such a way that surfactants orsurfactant mixtures of the type in question can be produced on anindustrial scale.

The first problem area concerns the sum total of measures which lead tooptimization of the nucleophilic substitution reaction illustrated bythe above equation. The second problem area concerns the removal andpurification of the valuable product formed from the reaction mixture.The following observations may be made in this regard:

The Nucleophilic Substitution

The preferred starting materials on the alkyl ether sulfate side arecorresponding components containing at least predominantly 6 to 18carbon atoms and, more particularly, 10 to 16 carbon atoms in the alkylgroup of the alcohol. Alcohols or mixtures of alcohols containing onaverage 12 to 14 carbon atoms in the molecule are particularly suitableas the basis of the class of surfactants in question. Suitable alcoholsare of both natural and synthetic origin; mixtures of such alcohols mayalso form the basis of the compounds in question. Suitable naturalstarting materials for the production of the alkyl ether sulfates, whichis not being claimed, are for example corresponding coconut oil and/orpalm kernel oil fatty alcohols which have been obtained by hydrogenatingreduction of the fatty acids or fatty acid methyl esters. The alcoholson which the alkyl ether sulfates are based are preferably saturatedcompounds, but if desired may also be olefinically unsaturated.

The alcohols used as principal starting material in the production ofthe alkyl ether sulfates are alkoxylated, ethylene oxide and/orpropylene oxide in particular being used in known manner for thealkoxylation of the alcohols. In the preferred embodiment, thecorresponding fatty alcohol alkoxylates have an average degree ofalkoxylation above 1 and preferably not more than 10, particularlysuitable average degrees of alkoxylation being between above 2 and 8.The HLB values of the nonionic surfactant components formed aredetermined in known manner by the length of the C chain distribution andthe degree of alkoxylation. The alkyl ether sulfates used as one of thekey reactants in the process according to the invention are formed byesterification of the alkyl ethoxylates with sulfuric acid, thecorresponding sodium salts generally being introduced into the reactionaccording to the invention in aqueous solution. It has been found inthis regard that 10 to 40% by weight aqueous solutions and, moreparticularly, 15 to 25% by weight aqueous solutions of the ethersulfates may be used with particular advantage. According to theinvention, however, it is also possible to use considerably moreconcentrated solutions, for example approximately 70% by weightsolutions, of the ether sulfates. The other reactant for thenucleophilic substitution reaction is sodium sulfite which is alsointroduced in aqueous solution. The sodium sulfite may be used as suchalthough it is also possible in accordance with the invention to use theless expensive sodium disulfite (Na₂ S₂ O₅) to save costs. It is knownthat Na₂ S₂ O₅ forms the following equilibrium in aqueous solution inthe presence of a base which, according to the invention, is preferablysodium hydroxide: ##STR1##

According to the invention, the alkali metal sulfite is used in astoichiometric excess in the reaction. A preferred upper limit to thisstoichiometric excess is at a molar ratio of sulfite to ether sulfate ofabout 5, the preferred lower limit being at about 1.5. A stoichiometricsulfite excess of about 1.5 to 3 mol per mol alkyl ether sulfate isparticularly suitable.

Another preferred embodiment of the invention is concerned with thepredetermined direction in which the reactant solutions to be reactedwith one another are added. In the more important embodiment of theinvention where the process is carried out in batches, the aqueousalkali metal sulfite is introduced first under the reaction conditions.The aqueous alkyl ether sulfate solution is introduced into the sulfitesolution, best with simultaneous mixing. More particularly, the alkylether sulfate solution is added in batches or preferably continuously atsuch a slow rate that a considerable proportion of the total reactiontime selected is required for the introduction of the alkyl ethersulfate. In the preferred embodiment, at least about 20% of the totalreaction time required is taken up by this addition.

In another preferred embodiment, the concentration of the reactants islimited so that surfactant contents of not more than about 40% by weightand, more particularly, up to about 25% by weight are established in theaqueous reaction mixture accumulating. The reaction is carried out in amildly alkaline pH range, more particularly at a pH value of from about7.5 to 10, "sodium-alkaline" conditions being particularly preferred.

By coordinating the reaction parameters with one another, the reactioncan be optimized towards the desired nucleophilic substitution reactionwith simultaneous limitation or minimization of the alcohol ethoxylatecomponent, preferably to values below 50% by weight and, moreparticularly, to values of at most about 30 to 40% by weight, based onthe total surfactant content. The particularly preferred processparameters are as follows: Reaction temperatures in the range from about180° to 200° C., pH values of the reaction mixture in the range fromabout 8 to 9 and molar ratios of sulfite to alkyl ether sulfate of 1.5to 2 at pressures of up to about 20 bar, preferably under "natural"pressure.

The surfactant mixtures formed may have alkyl ether sulfonate contentsof at least 60% by weight and preferably of at least about 70% byweight. The described choice of the delayed addition of the alkyl ethersulfate reaction component ensures that the unwanted qelation of thereaction mixture and, hence, the formation of crust-like deposits on theinner walls of the reactor are safely prevented. The reaction times aretypically between about 2 and 6 hours and preferably at least about 3hours per reaction batch.

ISOLATION OF THE SURFACTANT MIXTURE FORMED

The ether sulfonates, but preferably the mixture of ether sulfonates andalcohol alkoxylates thus formed has to be separated from the aqueoussalt solution in a following step. As already mentioned, it isparticularly important in this regard to ensure that the salts dissolvedin the aqueous phase only pass over into the surfactant mixture to beseparated in small quantities, if at all.

For this separation step, the teaching according to the invention uses ameasure of which neither the usefulness nor the effectiveness in theform described hereinafter was foreseeable.

Under carefully selected operating conditions, particularly in regard tothe operating temperature, it is possible to achieve substantiallyquantitative separation of the aqueous salt-containing phase and thesurfactant phase formed by extraction with water-insoluble alcohols. Atleast substantially water-insoluble alcohols of synthetic and/or naturalorigin preferably containing up to 14 carbon atoms and, moreparticularly, from 4 to 8 carbon atoms are suitable. Particularlysuitable extractants are amyl alcohols and/or hexanols of syntheticand/or natural origin which may be linear and/or branched. The followingobservations may again be made in this regard:

Extraction with the alcohol phase is carried out in one or more stages,preferably at temperatures of from about 50° to 100° C. and, morepreferably, at temperatures of from about 80° to 90° C. Suitable mixingratios of the reaction mixture to be extracted to the extractant arepreferably not more than about 30% by weight and, more particularly, inthe range from about 3.5 to 15% by weight. Extraction may be carried outin batches, semicontinuously or even continuously. Simple mixing of thephases to be contacted with one another is sufficient for effectiveextraction.

On completion of extraction, the aqueous phase and the alcoholic phasegenerally separate smoothly from one another, the alcoholic phase isremoved and is best subjected to further working up.

In the course of this working up, the alcohol used as extractant ispreferably at least partly recovered, for example by distillation, andseparated from the surfactant mixture formed. The alcohol recovered maybe reused in the next extraction phase.

For the purposes of the invention, however, it is particularly importantthat lower alcohols of the type used herein as extractants, moreparticularly the preferred C_(5/6) alcohols, are used in practice asso-called co-solvents for enhanced oil recovery. According to theinvention, therefore, it is not necessary at all to continue removal ofthe alcohol to very small residual contents in the surfactant mixture.Instead, the multicomponent surfactant/alcohol mixture still containingconsiderable proportions of alcohol may be directly used for theintended application. Preferred residual alcohol contents in thereaction product separated off are, for example, contents of up to about100% by weight, residual alcohol contents of from 15 to 35% by weight,based on the surfactant mixture, being particularly preferred.

The content of troublesome sulfate ions in the reaction mixture thusobtained is typically below 0.5% by weight and best below about 0.2% byweight and may even be kept at a maximum value of about 0.1% by weight.Such low contents of sulfate ions in the surfactant mixture areacceptable for the intended application.

EXAMPLES I. Starting Materials and Reaction Conditions for Three TypicalPilot Plant Trials (Reaction)

    ______________________________________                                        Starting material   Trial 1 Trial 2  Trial 3                                  ______________________________________                                        Texapon K 14 S; 25%                                                                           (kg)    310.5   414.2  --                                     (C.sub.12/14 ether sulfate; 3.6 EO)                                           LS 8 sulfate, 25%                                                                             (kg)    --      --     393                                    (C.sub.12/14 ether sulfate; 8 EO)                                             Sodium sulfite  (kg)    33.9    60.0   59                                     Water           (kg)    86.9    101.4  249.8                                  Surfactant content                                                                            (%)     18      18     14                                     mol SO.sub.3.sup.2-     1.5     2.0    3.0                                    mol ether sulfate                                                             pH                      9       9      9                                      Reaction temperature                                                                          (°C.)                                                                          210     200    180                                    Addition time   (h)     1       1      1                                      Reaction time   (h)     5       4      8                                      (incl. addition time)                                                         Yield of ether sulfonate                                                                      (%)     72.3    76.2   78.9                                   ______________________________________                                    

II. Starting Materials and Test Parameters for the Extraction Phase

    ______________________________________                                                 Trial 1   Trial 2    Trial 3                                         ______________________________________                                        Feed       RM* Trial 1 RM Trial 2 RM Trial 3                                  Extractant +           -          +                                           Hexanol    -           +          -                                           Pentanol (isomer                                                                         -           +          +                                           mixture)                                                                      Process batch                                                                            +           -          -                                           continuous                                                                    Feed       3.5         10         2.3                                         Extractant                                                                    ______________________________________                                         *Reaction mixture                                                        

III. Experimental Setup 1. Reaction

The syntheses of the ether sulfonates based on the corresponding ethersulfates were carried out in a 1 m³ pressure reactor with the followingtechnical data;

Stirred reactor, 1 m³, material 1.4539

Permitted operating pressure: 30 bar

In addition to the heating/cooling surface of the reactor wall, thereactor is equipped with an internal heating coil. 4-Stage slottedInternig mixer with a trapezoidal stirrer as the final element. Pistonmetering pump for introducing the ether sulfate solution.

2. Extraction

Batch: 1 m³ pressure reactor

Continuous: 3 m extraction limit column; internal diameter 5 cm, 50sieve plates; heatable through a double jacket

IV. Test Procedure 1. Reaction

The sodium sulfite solution adjusted with sulfuric acid to the desiredpH value is initially introduced into the reactor and heated to thereaction temperature, after which the ether sulfate solution with acorrespondingly adjusted pH value is introduced by means of the pistonpump (see addition time). After the reaction (see reaction time), thereaction mixture is cooled to a temperature below 100° C. andsubsequently worked up by extraction.

2. Extraction

The extraction tests were carried out with n-hexanol and pentanol(isomer mixture). The two alcohols are equivalent in their properties asextractants and in the extraction result.

The extraction tests were carried out at 80° C. because the depletion ofthe valuable product in the aqueous phase is promoted by elevatedtemperatures. In single-stage operation, ether sulfonate is depleted toless than 1% by weight in the raffinate so that there may even be noneed for multistage extraction. By addition of salts, such as NaCl, Na₂SO₄ or ammonium acetate, the coalescence behavior of the system can beimproved to such an extent that the phase separation times areshortened.

Test 1

The reaction mixture test 1 was continuously worked up in a 3 mextraction lift column with an internal diameter of 5 cm and 50 sieveplates. The throughput was 18 l/h, the ratio of feed to extractant 3.5and the temperature 80° C. n-Hexanol was used as extractant. An extracthaving the following composition was obtained:

    ______________________________________                                               Ether sulfonate:                                                                              19%                                                           Water:          21%                                                           Hexanol:        53%                                                           Impurities:*     7%                                                    ______________________________________                                         *Ether sulfate, nonionic surfactants, Na.sub.2 SO.sub.4                  

Test 2

The reaction mixture from test 2 was discontinuously worked up in asingle-stage extraction using pentanol (isomer mixture). To this end,the feed and extractant were initially introduced into a stirred reactorin a ratio of 10:1, heated to 80° C. and intensively stirred for 0.5 h(large phase interface). After phase separation, the two phases wereseparately run off. An extract having the following composition wasobtained:

    ______________________________________                                               Ether sulfonate:                                                                              29%                                                           Water:          40%                                                           Pentanol:       22%                                                           Impurities:*     9%                                                    ______________________________________                                         *Ether sulfate, nonionic surfactants, Na.sub.2 SO.sub.4                  

Test 3

The reaction mixture from test 3 was discontinuously worked up in asingle-stage extraction using n-hexanol. To this end, the feed andextractant were initially introduced into a stirred reactor in a ratioof 2.3:1 together with 2% NaCl, heated to 80°C. and intensively stirredfor 0.5 h (large phase interface). After phase separation, the twophases were separately run off and the organic phase was washed with 7%water and 1% NaCl.

An extract having the following composition was obtained:

    ______________________________________                                        Cl.sup.- :        0.2%                                                        SO.sub.3.sup.2- : --                                                          SO.sub.4.sup.2- : 0.05%                                                       LS8 sulfate:      1.2%                                                        LS8 sulfonate:    13.6%                                                       Nonionic surfactants:                                                                           4.4%                                                        ______________________________________                                    

We claim:
 1. The process of recovering oil from an oil reservoir,comprising preparing a surfactant mixture consisting essentially of analkyl ether sulfonic acid or salt thereof as a principal constituent andat most a substantially equal quantity of alkoxylated alcohol as aby-product of the process by reacting an alkyl ether sulfate with anaqueous alkali metal sulfite solution at a temperature of from about160° C. to about 220° C. a pH of from about 7.5 to about 10, and under apressure of up to about 20 bar, wherein said alkali metal sulfite ispresent in a stoichiometric excess with respect to said alkyl ethersulfate, and wherein said alkyl ether sulfate is added to said aqueousalkali metal sulfite solution with simultaneous mixing at a rate so thatabout 20% of the total reaction time is taken up by the addition of saidalkyl ether sulfate, and extracting the surfactant mixture formed with asubstantially water-insoluble alcohol at a temperature of from about 50°C. to about 100° C. whereby said surfactant mixture is substantiallyfree from sulfate salt, and flooding said oil reservoir with saidsurfactant mixture.
 2. The process as in claim 1 wherein said alkylether sulfate contains from 6 to 18 carbon atoms in the alkyl radicaland has an average degree of alkoxylation from about 1 to about
 10. 3.The process as in claim 1 wherein said alkali metal sulfite is initiallypresent in a molar ratio of up to about 5 with respect to said alkylether sulfate.
 4. The process as in claim 1 wherein said reacting stepis conducted at a temperature of from about 180° C. to about 200° C., ata pH from about 8 to about 9, and with a molar ratio of said alkalimetal sulfite to said alkyl ether sulfate in the range from about 1.5 toabout 2, whereby said surfactant mixture contains at least about 60% byweight of alkyl ether sulfonate, based on the weight of said surfactantmixture.
 5. The process as in claim 1 including separating saidwater-insoluble alcohol from said surfactant mixture.
 6. The process asin claim 1 wherein said surfactant mixture contains from about 15 toabout 35% by weight of said water-insoluble alcohol, based on the weightof said surfactant mixture.